Brainstorm ideas for movies. Ideas can come from anywhere and everywhere. Think of interesting characters whom you'd like to follow around for two hours; think of provocative situations that will provide dramatic moments. Seek inspiration from newspaper articles. Think of worlds unexplored; for instance, "Top Gun" takes place in the "world" of fighter pilots. Think of a genre in which you'd like to write, or a specific philosophy/idea/theme you want to impart to the world.
List all your ideas. Try to find where the conflict will come from for each idea. Conflict is the basis for all dramatic story-telling: If everything in movies was always hunky-dory and everyone got along, they wouldn't be very interesting. Each idea has unique possibilities for conflict.
Write log-lines for each idea. A log-line is a one or two sentence description of the movie that identifies characters, situation and conflict. As an example, the log-line for "Star Wars" might read: "An orphaned moisture farmer decides to help rescue a princess from the sinister clutches of the Intergalactic Empire." Log-lines should be short and to the point, and should encapsulate the movie.
Choose your favorite, or most-intruiging, log-line. This choice will dictate which screenplay you write. Keep old log-lines for further screenplays.
Identify the protagonist(s). The protagonist is the character in whom the audience will invest, the one we will follow throughout the course of the film and root for to achieve his or her goals. Write out a description and a background for the character, as well as what the character wants with respect to the screenplay and his own life.
Identify the antagonist(s). While antagonists work in direct contradiction to the protagonist's wants, do not necessarily think of the antagonist as a straight villain. Many antagonists are, especially in classically-structured films, but the antagonist could be anything: a person, nature, society or even the protagonist himself. Remember, antagonists have wants of their own, and those wants are not simply to make things difficult for the protagonist. Giving your antagonist actual wants provides for a more rounded and believable character.
Sketch the screenplay's world. What other characters occupy this world? Who are the protagonist's friends? Who are the antagonist's friends? What is the setting? Will the setting change? A well-planned, and well-executed, world will signal to studios and agents that you have the minutiae of your screenplay down pat.
Map out the story into three acts. All stories are comprised of three parts: the beginning, the middle, and the end. For screenplays, it's no different: the first act, usually around 30 pages, represents the beginning of the story, where we are introduced to the protagonist and all the characters, their situations, their wants and a hint of what the main conflict will be. The second act is the middle, typically about 60 pages, where we watch the protagonist and antagonist actively pursue their wants, and ends when we have a (temporary) answer to whether they achieved them. The third act is the end, the final 30 pages, where we see how the characters have changed and whether they can overcome the final obstacles toward attaining their goals. Follow the natural rhythms of story-telling.
Write a treatment. A treatment is a detailed synopsis of the screenplay written in prose. These can range anywhere from five to 50 pages, depending on the writer and the level of detail that the writer wants to include. Generally, a treatment hits the main story-beats; details important scenes, moments or sequences; and provides arcs for all the characters. A detailed treatment makes the actual writing of the screenplay much easier, as you have already created the blueprint. Revise the treatment as necessary until you feel ready to start writing.
Start with page one. The hardest part of the writing process is starting, and the second-hardest part is finishing. Find time to write, and learn what your writing habits are to make yourself more efficient.
Follow the treatment, but do not be afraid to veer off on tangents. Sometimes you will receive epiphanies while writing, and these epiphanies may alter what you've already written and/or what you've planned. If you truly believe your new idea will improve the script, will sharpen a character's wants or the main conflict, or provide for a particularly poignant moment, by all means go for it. If you are the least bit hesitant, jot the idea down, finish the screenplay, and consider the idea during the revision process.
Finish the screenplay. Do not talk yourself out of finishing. Do not let yourself take long breaks between writing. Do not accept writer's block. Getting the first draft out is all that's important, because once you have something to work with, you can alter it and mold it into whatever you want. Even if you think what you've written is complete garbage, soldier on. Writers can easily psych themselves out; do not let this happen.
Put the screenplay down for a length of time and forget about it. Once completed, give yourself a few days off to clear your head so, come revision time, you will approach the script with a fresh eye and an open mind.
Read through the script. Mark all grammar or formatting mistakes as you go. For some it helps to print the script out and read a hard-copy while others prefer reading on a computer. Whichever works best for you, choose that option. Make notes on where you think scenes are weak, character motivations are weak, or you know you can improve certain aspects of the script.
Look at the script as a whole to see what is working and what is not on the macro-level. What isn't working is the most important: fixing these weak points will create an invincible script with an airtight story and fascinating characters. Check for plot holes. Think about removing, adding or combining characters to allow the story to flow or breathe better. Sharpen character wants on the whole.
Create a beat-sheet. A beat-sheet is a type of outline where you list each scene as a beat and provide a one-sentence description of what happens in the scene. If you cannot explain what happens in the scene, it should be deleted. Scenes should illuminate characters, but they should also keep the story moving. If the scene does not achieve the latter goal, cut it out and think about other places to put the character moments. With a beat-sheet, you can better see the structure of the script and whether some scenes should be shuffled around order-wise or totally removed, as well as where scenes should be inserted. The beat-sheet provides a road-map for revisions.
Execute the beat-sheet. Go back through the script and rewrite it. Now, focus on the micro-level of the scene: sharpen character wants in each scene, and cut out anything that does not propel the story. Screenplays do not have the luxury of novels to meander or explore tangents for tens of pages at a time; screenplays range from 90-120 pages and must be focused.
Repeat these steps as necessary. Screenplays often undergo several drafts before being ready to show to producers, agents or managers. This is a normal part of the process, as each pass through the script refines it and cuts away extraneous information to reach the heart of the story. If you're unhappy with a certain draft, scrap it and rewrite.